Main menu

You are here

Pooideae

Pooideae. –This subfamily is the largest in the grass family consisting of 4200 species and 194 genera (Table 2) and occurs in all regions of the world with a temperate climate in high latitudes and altitudes. It includes some of the world’s most
prolific genera Festuca (635 species), Poa (544 species), Agrostis (219 species), Stipa (193 species), Bromus (160 species), Puccinellia (115 species), as well as many smaller and monotypic genera. Until the early 1980’s the Pooideae has probably been the subject of a greater number of classifications than any other
subfamily (Macfarlane, 1986; Macfarlane & Watson1980, 1982) and twelve such
classifications are mentioned and compared by Macfarlane & Watson (1982).
Up to this point all classifications were phenetically derived using morphology
and latterly anatomy; the most detailed of these is that of Macfarlane (1986),
where 157 genera, excluding the tribe Stipeae, are placed in seven tribes
within two supertribes Pooideae and Triticoideae. The classification of Clayton
& Renvoize (1986) included 153 genera in 10 tribes with their relationships
presented in three phylogenetic diagrams. An attempt to undertake a cladistic
study based on morphology in the Pooideae (Kellogg & Watson 1993), based on
189 genera, resulted in a lack of a hierarchy equivalent to traditional tribes.
Reasons given for this lack of resolution were a rapid burst of evolution in
the tribe, intergeneric gene flow and parallellism of morphological characters.
Early molecular studies, based on 33 species of 28 genera (Soreng et al., 1990)
did, however, produce a topology onto which 5 traditional tribes of the
subfamily could be superimposed. A cladistic study based on both 601 molecular
characters (chloroplast DNA) and 67 morphological characters (Soreng &
Davis, 2000), using 79 genera and 101 species of the Pooideae resulted in a
topology that blurred the distinction between the tribes Aveneae and the Poeae
to the extent that it was recommended that the barrier between these tribes be
abandoned, a proposal first suggested on non-phylogenetic principles (Tzvelev,
1989). Within this study the Bromeae, Triticeae, Meliceae, Brachypodeae and
Stipeae were shown to be monophyletic, although the sample size was far less
than that for the Aveneae/Poeae complex. Some of these results were
substantiated by Davis and Soreng (2007) and Soreng at al. (2007), and in the latter
paper the Poaeae sens. lat. (Aveneae/Poeae complex) are classified into 21
subtribes on the basis of their position in a cladogram derived from
morphological data and data from nucleotide sequence variation. Based on only
57 species and with no attempt to allocate diagnostic characters to the
subtribes, I am of the opinion the proposal of a new classification on the
basis of the data presented is premature. A similar study of the phylogenetic
relationship within the Aveneae/Poeae complex based on matK chloroplast sequence data (Doring et al 2007) verified the
undefined boundary between the genera traditionally allocated to these these
tribes and that a new generic circumscription of this group is probably
required before applying a subtribal classification.

Although there has been a concentration of work in specific parts of the world, in particular North America (Barkworth 1990, 1993), Australia (Jacobs & Everett, 1996), south west Asia (Freitag, 1975, 1985) and Asia (Gonzalo et al. 2011) there is no published work that includes a classification of all the genera listed, although Romaschenko et al. (2011) includes all genera except Anemanthele. Keys have been published that discriminate between some of the genera, but the lack of easily discernible morphological characters would explain a trend by some workers not to accept some genera. For example Clayton and Renvoize (1986), who only include 9 genera in the tribe Stipeae, place the genera Jarava, Achnatherum. Macrochloa, Ptilagrostis and Anemanthele in synonymy with Stipa, and present Kew researchers are also reluctant to accept Austrostipa (Cope 2008.). A molecular phylogeny of the tribe Stipeae (Jacobs et al. 2007) shows good support for the monophyly of only three genera - Nassella, Hesperostipa and Piptochaetium. Howeverthe recent molecular work of Romaschenko et al. (2008, 2008a) has seven major lineages in the tribe, and that of Barber et al. (2009) finds a problem distinguishing Nassella from Jarava based on molecular data. These papers indicate that the resolution of generic boundaries in the Stipeae is some way from being resolved. Although these authors discuss some morphological characters the latter appear to be of secondary consideration and they do not provide diagnostic morphological keys to the lineages. A new genus Pappostipa with 23 species is elevated from Stipa subg. Pappostipa (Romaschenko et al., 2008) without a clear indication as to how the genus differs morphologically from Stipa.